问题描述
我最近在JavaScript中发现 2 == [2]
。事实证明,这个怪癖有几个有趣的后果:
I recently discovered that 2 == [2]
in JavaScript. As it turns out, this quirk has a couple of interesting consequences:
var a = [0, 1, 2, 3];
a[[2]] === a[2]; // this is true
同样,以下作品:
var a = { "abc" : 1 };
a[["abc"]] === a["abc"]; // this is also true
更奇怪的是,这也有效:
Even stranger still, this works as well:
[[[[[[[2]]]]]]] == 2; // this is true too! WTF?
这些行为似乎在所有浏览器中都是一致的。
These behaviors seem consistent across all browsers.
知道为什么这是一种语言功能吗?
Any idea why this is a language feature?
以下是这个功能的更为疯狂的后果:
Here are more insane consequences of this "feature":
[0] == false // true
if ([0]) { /* executes */ } // [0] is both true and false!
var a = [0];
a == a // true
a == !a // also true, WTF?
这些例子来自jimbojw 成名以及。
These examples were found by jimbojw http://jimbojw.com fame as well as walkingeyerobot.
推荐答案
您可以在ECMA规范中查找比较算法(ECMA-262的相关章节,第3版针对您的问题:11.9.3, 9.1,8.6.2.6)。
You can look up the comparison algorithm in the ECMA-spec (relevant sections of ECMA-262, 3rd edition for your problem: 11.9.3, 9.1, 8.6.2.6).
如果将所涉及的抽象算法转换回JS,那么在评估 2 == [2]时会发生什么? / code>基本上是这样的:
If you translate the involved abstract algorithms back to JS, what happens when evaluating 2 == [2]
is basically this:
2 === Number([2].valueOf().toString())
其中 valueOf()
对于数组返回数组本身,单元素数组的字符串表示形式是单个元素的字符串表示。
where valueOf()
for arrays returns the array itself and the string-representation of a one-element array is the string representation of the single element.
这也解释了第三个例子为 [[[[[[[2]]]]]]]。toString()
仍然只是字符串 2
。
This also explains the third example as [[[[[[[2]]]]]]].toString()
is still just the string 2
.
As你可以看到,有相当多的幕后魔术,这就是为什么我一般只使用严格的相等运算符 ===
。
As you can see, there's quite a lot of behind-the-scene magic involved, which is why I generally only use the strict equality operator ===
.
第一个和第二个例子更容易理解,因为属性名称总是字符串,所以
The first and second example are easier to follow as property names are always strings, so
a[[2]]
相当于
a[[2].toString()]
只是
a["2"]
请记住,在任何数组魔法发生之前,偶数数字键也会被视为属性名称(即字符串)。
Keep in mind that even numeric keys are treated as property names (ie strings) before any array-magic happens.
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